In the optical communications space, techniques used to detect data modulated onto an optical communications signal may be broadly group into two classes, namely “direct” detection and “coherent” detection. In “direct” detection techniques, the optical signal is made incident on a photodetector. The electrical current appearing at the photodetector output is proportional to the square of the optical E-field. Data modulated onto the optical signal using an amplitude-modulation scheme, such as On-Off Keying (OOK) can thus be detected by analysis of the photodetector output current. Direct detection techniques have advantages in terms of low cost, and high reliability for On-Off Keying (OOK) based modulation schemes. As a result, the majority of optical receivers currently used in optical communications networks are based on direct detection.
In “coherent” detection techniques, the optical signal is mixed with a strong, narrow-line-width, local oscillator signal by an optical hybrid, and the combined signal made incident on one or more photodetectors. In some systems, the inbound optical signal is first split into orthogonal polarizations, and each polarization processed by a respective optical hybrid. In-phase and Quadrature components of each polarization can be detected using respective photodetectors positioned to receive corresponding signals output by the optical hybrid. The frequency spectrum of the electrical current appearing at the photodetector output(s) is substantially proportional to the convolution of the spectrum of the received optical signal and the local oscillator, and contains a signal component lying at an intermediate frequency that contains the data. Consequently, this “data component” can be isolated and detected by electronically filtering and processing the photodetector output current.
Coherent detection receivers offer numerous advantage over direct detection receivers, many of which follow from the fact that coherent detection techniques provide both phase and amplitude information of the optical signal. As such, more robust modulation schemes, such as phase shift keying (PSK), differential phase shift keying (DPSK) and quadrature phase shift keying (QPSK) can be used.
As is well known in the art, accurate recovery of a Clock signal from the received optical signal is essential for all digital signal processing techniques, and is fundamental in all digital signal receiver systems. Typically, the clock signal is recovered from the photodetector output current. For example, quadrature coherent receivers are described by R Noé, in: “Phase Noise-Tolerant Synchronous QPSK/BPSK Baseband-Type Intradyne Receiver Concept With Feedforward Carrier Recovery”, Journal of Lightwave Technology, Vol. 23, No. 2, February 2005, and “PLL-Free Synchronous QPSK Polarization Multiplex/Diversity Receiver Concept with Digital I&Q Baseband Processing”, IEEE Photonics Technology Letters, Vol. 17, No. 4, April 2005; and by Y. Han et al. in “Coherent optical Communication Using Polarization Multiple-Input-Multiple-Output”, OPTICS EXPRESS Vol. 13, No. 19, pp 7527-7534, 19 Sep. 2005.
FIG. 1a schematically illustrates the system of Noé (Supra, April 2005). As may be seen in FIG. 1, an optical signal received through an optical link 2 is divided by a polarization beam splitter 4 into orthogonal polarizations (nominally referred to as X and Y polarizations in FIG. 1), which are then mixed with a local oscillator (LO) 6 through a quadrature 90° optical hybrid 8. The composite optical signals appearing at the output of the optical hybrid 8 are made incident on a set of photodetectors 10 to generate analog electrical signals respectively corresponding to real (Re) and imaginary (Im) parts of each polarization. These analog signals are then sampled at the symbol rate by respective Analog-to-Digital (A/D) converters 12 to generate digital sample streams of each of the real (Re) and imaginary (Im) parts of each polarization. The digital samples are then supplied to a 1:M DEMUXer 14, which splits the data path into M parallel sample streams having a lower sample rate (by a factor of M), each of which is supplied to a respective processing module 16. Within each processing module 16, an inverse Jones matrix that models the polarization performance of the optical link is used to compensate polarization distortions. The polarization compensated samples can then be decoded for data recovery.
In the system of Noé (April 2005), clock recovery is performed using either: a clock recovery block 18 inserted into the data path between the photodetectors and the A/D converters (FIG. 1a), or alternatively using an intensity modulation direct detection receiver 20 as shown in FIG. 1b. Recovering the clock signals from the electrical I and Q signals generated by the photodetectors, as shown in FIG. 1a, is beneficial in that it keeps all of the received optical power within the main data path, and at the same time makes both amplitude and phase information of the received optical signal available to the clock recovery circuit. However, this solution renders the system extremely sensitive to polarization impairments. The use of a direct detection receiver 20 for clock recovery, as shown in FIG. 1b, avoids problems associated with the polarization sensitivity of the coherent receiver. However, this solution diverts at least a portion of the energy of the received optical signal out of the data path, and is vulnerable to severe chromatic dispersion and polarization impairments at least in part due to inter-symbol interference (ISI).
For example, consider a scenario in which first order PMD on the optical link has a magnitude equal to one half of a symbol period. In this case, the received optical signal contains a mixture of two versions of the transmitted data signals, separated by a half symbol differential delay. Interference between the two versions can prevent the clock recovery circuit from successfully achieving a phase/frequency locked state. Indeed, in this example, when the signal power is equally split between the two modes of the PMD, the recovered clock tone goes to zero. In real-world networks, the amount of power in each mode varies with time. Each time the amount of power in one mode becomes greater than that of the other mode, the phase of the recovered clock jumps by 180 degrees.
The polarization impairments are generally time varying, with speeds as high as tens of kilohertz. This means that the phase of the recovered clock can be moved about by the polarization impairments. A clock recovery that was locked can lose lock. This does not provide a reliable communication link.
The above noted problems, in respect of both systems, are compounded for polarization-division multiplexed signals, in which each transmitted polarization contains a respective different data signal. Neither of the techniques suggested by Noé offers a robust solution for clock recovery from highly distorted optical signals of the type encountered in “real-world” optical communications networks.
Accordingly, techniques enabling clock recovery from a received optical signal, in the presence of severe distortions, remains highly desirable.